The ONLY person who can unlock the watch is the person who locked it. You need to reach out to the seller and give them this link --> What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - Apple Support
If you can't reach them, don't know them or they are unwilling to remove the watch from their account, you own a useless wrist ornament, you can never use. Apple will NOT help you. We cannot help you.
Either get the previous owner to remove the watch from their account. Or get a refund. Or recycle the watch responsibly, as it is totally useless to you.
Read this as well --> The All Too Common SAD Reality of Buying … - Apple Community